Specifically,
the redirect during checkout
is caused by a problem with Internet Explorer.
Click here for more detailed information, and a fix for the problem.
Unfortunately, Microsoft has not yet issued the fix to this problem
as an automatic Windows update. This means that the
currently available fix is one which will require users to manually
install it into their Internet Explorer browsers. While it is unclear
exactly when the fix will be made available in an automatic update,
it's at least more likely that it will when a new service pack update
is released.